Electron emitters are devices that emit electrons when subjected to external stimuli. Field emitters are devices that produce electrons under the influence of an electric field. Field emitters are used as an electron source in a variety of applications such as e-beam lithography, scanning electron microscopy, electron accelerators, X-ray sources, etc. Electron emitters already find applications in electron-linear accelerator (linac) factories for molybdenum-99 (Mo-99) production for nuclear medicine to rule out weapons grade uranium from the production cycle, or compact bright inverse Compton sources for basic science research and semiconductor lithography.
Conventional field emitters are generally shaped in the form of sharp tips (e.g., wires, cones, pyramids, etc.) having tip diameter or otherwise cross-section in the order of few tens of nanometers. Such conventional field emitters rely on micro- or nano-lithography and additional transfer steps for fabrication. These make it cumbersome, e.g., to scale field emitter size to tens or hundreds of millimeters or to use substrates of different form-factors and/or curvatures. Many industrial applications require normal conducting or superconducting radio frequency (RF) systems to deliver beam power of 10 kW to 100 kW at electron energy of 10 MeV to 50 MeV for which currents of 1 mAmp to 10 mAmps are needed. Many field emitters are capable of producing such currents. However, accelerator electron injectors pose some challenging and unique requirements. For instance: 1) requiring mechanical and electrical strength which does not poison niobium superconducting resonators; 2) low turn-on electric fields and high rise of current-voltage characteristic to yield significant currents in low gradient conditions; and 3) simplicity of a field emitter is also a key matter for servicing and small downtime.
Similarly, photocathodes are electron emitters that emit electrons when exposed to photons due to photoelectric effect. Photocathodes are a key component of photo-injectors in synchrotron sources, free electron lasers, linacs and ultrafast electron systems for imaging and diffraction. Conventional photocathodes have either low efficiency and are stable against air exposures (e.g., copper) or have high efficiency but are unstable against poor vacuum (>10−9-10-8 Torr) and air exposures (e.g., alkali based materials). Such high efficiency alkali photocathodes thus have to be maintained at ultrahigh vacuum (i.e., significantly less than 10−9 Torr) for operation. Exposure to higher pressures degrades alkali photocathodes and halts photoemission.